This invention relates to a microelectromechanical systems (MEMS) device and its method of manufacture. More particularly, this invention relates to a material and process for bonding MEMS wafers with a protective lid wafer.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be a cantilevered beam which connects one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable cantilevered beam switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
Because the MEMS devices often have moveable components, such as the cantilevered beam, they typically require protection of the vulnerable moveable portions by sealing the devices in a protective cap or lid wafer, to form an encapsulated device. Furthermore, if the MEMS device is intended to operate in a particular environment, for example, a MEMS switch handling high voltages may be required to operate in an electrically insulating environment, and thus the MEMS switch may be encapsulated with an electrically insulating gas. In order to prevent the preferred gas environment from leaking out over the lifetime of the switch, the environment may need to be sealed hermetically when the lid wafer and the device wafer are bonded.
The lid wafer may be secured to the device layer by some adhesive means, such as a low outgassing epoxy. To fabricate the encapsulated MEMS device, a device wafer upon which the MEMS devices have been fabricated using the batch processing techniques, is placed against the lid wafer. Adhesive has been placed on the device wafer or the lid wafer, or both. The device wafer is pressed against the lid wafer, and heat is applied to melt or cure the adhesive. After curing, the device wafer and lid wafer assembly is generally sawed to singulate the individual devices.
Many adhesives such as epoxies, cements and glues are liquid during application, and only harden upon curing. Alternatively, an adhesive such as a solder or metal can be melted until it flows, and then cooled to harden. In either case, the adhesive may need to be a liquid at some point in order to accommodate variations in the surfaces of the lid wafer and the device wafer and securely bond the surfaces. The liquid will, in general, flow outward from the bond region during assembly, such that a rigid feature or standoff may need to be provided in the lid wafer or device wafer to define a minimum separation between the lid wafer and the device wafer. The separation may be that required to accommodate the height of the MEMS device, as well as some extra room to accommodate its movement.
FIG. 1 shows an example of a portion of a prior art lid wafer for forming the protective lid for a MEMS device and having a standoff to define the minimum separation between a lid wafer and a device wafer. The MEMS device 140, is shown only schematically in FIG. 1, because the details of the MEMS device 140 are not essential to the understanding of this invention. The MEMS device 140 has been previously formed on device wafer 150. The lid wafer 160 is processed to form a recessed region 170. This recess is sufficiently deep to provide clearance for the MEMS device 140 and its movement. The recess 170 may be formed, for example, by reactive ion etching the surface of the lid wafer 160, after appropriate patterning with photoresist. During formation of recess 170, the mechanical standoffs 120 may be formed by protecting these areas from the reactive ion etching process. Alternatively, standoffs 120 may be formed by depositing a material, such as a metal film, in these regions.
The device wafer 150 is generally bonded to the lid wafer 160 with an adhesive bond, using a wafer bonding tool (not shown in FIG. 1). To achieve the adhesive bond, a layer of adhesive 110 is deposited on a cap or lid wafer 160, or on the device wafer 150, around the perimeter of the MEMS device 140. The device wafer and lid wafer may be aligned so that the standoff features 120 are properly placed with respect to the MEMS devices 140, and clamped together to form the wafer assembly 100. The wafer assembly 100 may then mounted in the wafer bonding tool. The assembly 100 may then be heated to liquefy or cure the adhesive 110. Because of the pressure, the liquid adhesive 110 flows outward from the bond region, allowing the device wafer 150 and the lid wafer 160 to come within a minimum distance defined by the standoffs 120. The assembly 100 is allowed to remain stationary until the device wafer 150 is permanently bonded to the lid wafer 160. The assembly 100 is then cooled and removed from the wafer bonding tool. The devices are subsequently singulated, to form the individual encapsulated MEMS devices,
Using the approach illustrated in FIG. 1, the lid wafer 160 must be processed to form the standoffs 120 before alignment and bonding to the device wafer 150. This processing may take the form of one or more additional photolithography steps, such as deposition of photoresist, patterning of the photoresist, and followed by etching of the lid wafer 160. These additional steps add cost and complexity to the formation of the encapsulated MEMS device.